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100 PCB007 MAGAZINE I FEBRUARY 2021 overall process is vital when making improve- ments. You need to start by establishing what the current capabilities of your solder mask pro- cess are. A helpful way to do this is to create one or more test panel designs, incorporat- ing as many of the performance characteris- tics you want to benchmark as possible, and including as many of the possible variables that you suspect will have an impact on the results. Your test panel needs to be designed to push the limits of what you believe your current solder mask capabilities are. is way you can see a clear, measurable cutoff point in your current process so that when you make changes later, you can effectively tell what the impact is. For example, if you can only reliably clear holes down to 8 mil finished, you want to make sure your test panel design has finished hole sizes above and below that. Remember, for this example, panel thickness also plays a role so you will want to make panels of differ- ent overall thickness as well as with plated and non-plated holes. In another example, if you are looking to improve the minimum solder mask web that you can maintain, you would want to include a pattern with decreasing web widths and var- ious configurations across the panel to see what your current process can do consistent- ly. Where there are many additional variables influencing web retention, your test panels should also include different diameter holes so you can correlate hole clearing with web retention, because being able to reproduce small webs is useless if you can't clear the holes. You should also use the same test pattern with multiple Cu trace heights and spacing, be- cause you will likely see a correlation of circuit heights and solder mask thickness to the mini- mum web retained. Your results will also differ by type and color of solder mask used. Again, the same test vehicle can be designed to incor- porate coating uniformity, solder mask cover- age over different circuit configurations, solder mask registration, etc. Once you have identified the areas you want to improve, and established the baseline of what your current process is capable of, you can then alter individual variables within your process to evaluate what impact they have on your final results. Remember, a change you make to one step of the process can impact sev- eral later process steps. An example of this ef- fect might be a simple change in squeegee an- gle at solder mask coat. Such a change may im- pact how much solder mask is deposited into the holes, which can then increase phototo- ol sticking, increase exposure times, and in- crease solder mask residue on the phototool as well as create solder mask hole clearing issues at developing, resulting in increased develop- er dwell times and solder mask web retention difficulties. So, before you start down the path of process improvement, it is best to have a good, mea- surable system in place to understand what your process can do before making modifica- tions. Otherwise, you will not be able to tell if the changes are improving the process or just fixing one issue while creating another issue down the line. PCB007 Bob MacRae is the Eastern RegionalSales Manager for Taiyo America, Inc. Your test panel needs to be designed to push the limits of what you believe your current solder mask capabilities are.

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